Before rolling, it is first necessary to bring the metallic products to a sufficiently high temperature, and secondly, it is desirable for this temperature to be as uniform as possible throughout the mass of such products. The second objective is especially difficult to achieve in the case of large size products such as slabs.
In conventional industrial flame furnaces, intended for reheating slabs prior to rolling, whether these are pusher (slabs joined) or walking beams (slabs separated) type furnaces, the slabs move on a bed plate constituted by support elements arranged parallel to the direction of movement of the slabs. These support elements may be constituted solely by fixed slides (in pusher type furnaces) or by one group of fixed bars and another group of movable beams, the latter assuring the displacement of the slabs to the interior of the furnace (in walking beams type furnaces). At the region of contact between a slab and the support elements, there are usually regions which are colder than the slab as a whole, and these can sometimes spread through the entire thickness of the product. These colder regions are superficially recognizable by a coloration darker than the rest of the product, and for this reason they are generally referred to as "skid marks." Such a phenomenon arises particularly in furnaces intended for reheating thick products where heating takes place not only from the vault but also from the bottom of the furnace, beneath the products, because, in the first place, the support elements must be cooled interiorly and, in the second place, the area of contact between the slabs and the support elements is not directly exposed to the heat, which is transmitted for the most part by radiation.
The presence of these skid marks is the cause of local temperature gradients sufficiently great to cause metallurgical defects in the product, and leads to extra thickness during final rolling.
It has been attempted to diminish such defects by acting, at the level of the rolling mill, on the water jets intended for the removal of calamine from the products in passage. The cooling effect resulting from this can be adapted for the purpose of reducing skid marks, e.g., by cooling the product with the exception of these coldest regions. However, it was soon recognized that this method, which is inconvenient to carry out, produces the desired results only with difficulty, and it is therefore seldom used.
Another solution consists of causing the reheated products to remain in the non-heated terminal area of the furnace (the thermal equalization zone), so as to allow a better temperature distribution in the entire product mass. However, this process is quite slow, and homogenization is achieved only at the cost of lowered productivity or of increased size of the installation, and requires considerable added energy expenditure.
A further solution, which has not yet, to applicant's knowledge, been used industrially, consists of placing in the rolling conveyor at the furnace outlet one or more magnetic sliding field inductors for local heating of the slabs at the locations of the skid marks (see French Patent No. 76 35635). However, this inductive heating solution, while in itself rather neat, is based on discontinuous operation of the inductors, and hence requires a system of detecting the skid marks, and interruption of the movement of the slabs when the skid marks are located above the inductors.